Image dissector photomultiplier tube

ABSTRACT

1,059,807. Photo-electric direction finders. BENDIX CORPORATION. July 22, 1965 [July 29, 1964], No. 31281/65. Heading H4D. [Also in Division H1] In a method of controlling the attitude of a space vehicle relative to a light source, e.g. a star or planet, an image of the light source is projected on to the photo-cathode of an image dissector tube, to produce a stream of photoelectrons which is focused and deflected over an aperture, so that electrons from a small area of the photo-cathode pass through this aperture, and through an electron multiplier tube, to a collector electrode. The output signal is combined with reference output data from one or both of the horizontal and vertical deflection systems to generate an error signal representative of the angular position of the vehicle relative to the light source. The error signal may be used to maintain one axis of the vehicle in a desired attitude relative to a fixed light source, or may be fed to servomotors which position gimbals to point the optical axis of a tracker to a light source. -

United States Patent C) 3,366,830 IMAGE DISSECTUR PHOTOMULTEPLIER TUBE Wiliiam R. Polye, River Edge, NJ, assignor to The Bendix Corporation, Teterhoro, Nl, a corporation of Delaware Filed July 29, 1964, er. No. 385,878 7 Claims. (U. 315-11) ABSTRACT OF THE DESCLOSURE A photomultiplier tube comprising an envelope enclosing a photocathode for photocmitting electrons in response to a photoimage thereon. The photo-electrons are directed by an electrostatic field to an electron multiplier formed of a tube-like member having its inner walls coated with secondary electron emissive material. A potential difference is applied to the ends of the electron multiplier without applying potentials in between to minimize the number of conductors extending through the envelope. A collector provides an output representing the photoimage on the cathode.

The present invention relates to an image dissector photomultiplier tube suitable for use in optical tracking systems such as star trackers and planet trackers.

The image dissector photomultiplier tube of the present invention includes a photocathode which emits a stream of electrons in response to a photoimage impinging thereon. In general, the photoimage will not fall on the center of the photocathode, but rather, it will be displaced some radial distance on the photocathode thereby producing an off-axis electron stream. A scanning system is provided which causes a magnetic field to be applied to the electron stream to periodically deflect the stream across an aperture which defines the entrance to an electron multiplier. Electrons passing through the aperture enter the electron multiplier which functions to greatly amplify the number of electrons. The output from the electron multiplier impinges upon a collector electrode producing an output signal. This output signal when combined with reference output data from the scanning system is capable of being processed to provide an error signal having a polarity and magnitude corresponding to the direction and extent the photoimage is displaced from a reference axis on the photocathode surface.

The scanning is such that the electron multiplier views only a small segment of the photocathode at any one time. By viewing only a small segment at any one time, a minimum of background is admitted to the electron multiplier providing a maximum signal to noise ratio.

Scanning systems associated with conventional photomultiplier tubes are basically of the electromechanical type and require rotation or vibration of reflectors, prisms, reticles or lenses. Such systems have difiiculty maintaining precise operation in a space environment for extended periods.

The scanning system used in the present invention has no moving parts. In this method of scanning, the entire optical system is fixed, and scanning is accomplished by magnetically shifting the stream of the electrons emitted by an image at the photocathode of the photomultiplier.

In hitherto existing devices, the electron multiplier of conventional photomultiplier tubes has taken the form of a plurality of carefully spaced dynodes requiring numerous electrode connections through the vacuum envelope and requiring a wide diameter tube to accommodate the spaced dynodes. In addition, the dynodes are operated at increasing potentials and an external voltage divider is required to permit proper potential distribution.

The electron multiplier employed in the present invention requires only one additional electrode connection while the conventional multiplier requires sixteen. This accounts for a vast increase in reliability. The electron multiplier employed in the present invention also obviates the need for numerous glass to metal seals required for the numerous electrodes extending through the envelope in a conventional photomultiplier. Further, the photomultiplier tube of the present invention is greatly reduced in size relative to conventional photomultiplier tubes because the electron multiplier employed in the tube can be made a few thousands of an inch in diameter. The resulting decrease in size greatly increases space vehicle applications of the tube.

An object of the present invention is to provide an improved image dissector photomultiplier adapted for use in a star tracker system.

Another object of the present invention is to provide an image dissector photomultiplier tube which views only a small segment of its field of view at any one time thereby eliminating background disturbances and providing a high signal to noise ratio.

Another object of the present invention is to provide an image dissector photomultiplier tube which uses electronic scanning means having no mechanical moving parts making it capable of precise operation for extended durations in a space environment.

Another object of the present invention is to provide an improved image dissector photomultiplier tube employing an electron multiplier having a diameter of minimum dimension which minimizes the cross-sectional tube area.

Another object of the present invention is to provide an image dissector photomultiplier tube using an electron multiplier electrically connected by only two leads thereby minimizing the number of internal welds and special surfaces on the tube and providing a vast increase in reliability.

Another object of the present invention is to provide an image dissector photomultiplier tube employing an electron multiplier comprising a continuous surface of high resistivity with a potential diiference applied between the ends of the surface to produce a uniform voltage gradient across the surface thereby eliminating the need for a voltage divider network associated with conventional electron multipliers.

Another object of the present invention is to provide an image dissector photomultiplier tube using an electron multiplier having a minimum size and a minimum number of components, and a minimum number of electrical connections.

These and other objects and features of the invention are pointed out in the following description in terms of the embodiment thereof which is shown in the accompanying drawings. It is to be understood, however, that the drawings are for the purpose of illustration only and are not a definition of the limits of the invention, reference being had to the appended claims for this purpose.

In the drawings:

FIGURE 1 is an axial section through an image dissector photomultiplier tube constructed in accordance with the present invention.

FIGURE 2 is a sectional view taken on line 22 of FIGURE 1.

Referring to FIGURE 1, an evacuated tubular envelope 1 is shown. Tube 1 is closed at its ends by end Walls 3 and 5 which extend transverse to the longitudinal axis 7 of the tube 1. End wall 3 is transparent and optically polished. A photocathode, indicated by reference numeral 9, is coated on the internal surface of end wall 3, The photocathode 9 consists of a fine deposit of any well known photo electric material, such as a mixture of antimony and cesium. Light impinging upon the photocathode 9 causes electrons to be. emitted from its inner surface producing an electron image spatially identical to the photoimage on the outer surface of photocathode 9.

An electron lens is supported within the envelope 1. The electron lens includes a cathode sleeve 11 formed by a metallic film deposited upon the inner wall of tube 1 and having its upper edge conductively connected to the photocathode 9. Electrical contact is made to cathode sleeve 11 and in turn photocathode 9 by means of a lead pin 12 sealed through the wall of tube 1. The coating 11 is connected to lead pin 12 by a metallic finger 13 which is integral with sleeve 11 and spring biased against lead pin 12.

The electron lens also includes an anode sleeve 14 which coaxially extends a short distance inside the cathode sleeve 11. Electrical contact is made to anode sleeve 14 through a masking electrode 20, a metallic finger 18 and a lead pin 16. Metallic finger 18 is integrally secured to masking electrode 20. Masking electrode 20 supports an end of the anode sleeve 14 and has -a central aperture 21 to allow a stream of electrons emitted from a small segment of the photocathode 9 to pass therethrough. An apertured member 22 is fitted into an opposite end of anode sleeve 14 from the masking electrode 20. The aperture in member 22 is concentric and in alignment with aperture 21 but it has a much large diameter.

A potential difference of approximately 150 volts is maintained between cathode sleeve 11 and anode sleeve 14 to direct a convergent focusing field on the electron beam emitted by photocathode 9 and thereby form a sharply defined electron image in the plane of masking electrode 20.

The sizes of the sleeves 11 and 13, and positions thereof, are somewhat critical and can be determined by experimental techniques to meet design requirement.

Magnetic deflection coils 23 and 24 energized by scanning generator 25 and magnetic deflection coils 27 and 29 energized by scanning generator 31 are spaced 90 from one another about the envelope 1. Coils 23 and 24 provide magnetic fields for deflecting an electron stream emitted by photocathode 9 along the axis designated as the x axis in FIGURE 2. Coils 27 and 29 provide magnetic fields for deflecting the electron stream along the axis designated as the y axis in FIGURE 2. The magnetic deflection coils 23, 24, 27, and 29 deflect the electron stream along the y and x axes causing small segments of the photocathode 9 to be presented to aperture 21 in a progressive manner such that the entire photocathode surface is periodically scanned. By viewing only small segments at any one time, a minimum of background is admitted to aperture 21 providing a maximum signal to noise ratio.

. The electrons arriving at aperture 21 constitute a relatively weak signal and so, an electron multiplier tube 32 is provided to amplify them. The electron multiplier tube 32 may be of the type shown and described in U.S. Patent No. 3,128,408, granted Apr. 7, 1964 to G. W. Goodrich et al. and assigned to The Bendix Corporation, the same assignee as the present invention.

The inside surface 33 of electron multiplier tube 32 is resistive and has secondary emissive properties. Electrical contact to this resistive surface 33 is made by coating the ends of the tube with a conductive paint and attaching thereto lead pins 16 and 35. Upon application of a potential difference of 1000-2000 volts between the ends of the tube 32, current flows through the inner resistive surface producing an electric field in an axial direction through the region defined by the tube 32. Electrons or particles of suitable energy passing through aperture 21 and entering the electron multiplier tube 33 are multiplied through secondary emission before they emerge from the output end of the tube. The multiplier tube 33 is slightly curved so that no electrons can move straight through the tube without electrons creatings tube wall collisions.

The operation of the electron multiplier tube 32 is such that when primary electrons from the photocathode 9 strike the inner surface 33 of the tube 32 a secondary emission of electrons is generated with a small transverse velocity which will tend to carry it across the tube 32 While the longitudinal electric field accelerates it down the tube 32. By proper proportioning of the tube diameter, a sufficient amount of energy is imparted to a typical electron so that it will, on the average, generate more than one secondary emission of electrons upon collision with the opposite wall of the tube 32. Thus, a cascading action is instigated which can produce electron gain up to 10 Multiplier 32 replaces the multiplicity of dynodes upon which more conventional detectors rely for electron multiplication. In standard detectors, diflerent voltages must be applied to each dynode. Multiplier 32 removes this requirement, along with the associated voltage divider hardware. It also obviates the need for separate glass-tometal seals through the envelope for each dynode as required for conventional electron multipliers.

The electron stream output from electron multiplier 32 impinges upon the collector electrode 41 which is connected through lead pin 43, conductor 45, and load resistor 47 to ground. The output signal developed across load resistor 4'7 is then applied to a signal processing system 49 which combines it with reference output data from scanning generator 25. These signals are processed by signal processing system 49 such that an error signal E is produced, having a polarity and magnitude corresponding to the direction and extent the photoimage is displaced from the y axis. The error signal may be used to control a vehicle control system to position the vehicle so that the photoimage moves to the center of the x axis thereby maintaining one axis of the vehicle in a desired attitude relative to a fixed light source.

In other applications, the error signals are suitably amplified and fed to servomotors which position gimbals to point the optical axis of a tracker to a light source.

The signal processing system 49 may be of the type shown and described in copending US. patent application Ser. No. 385,902, by Edward A. Chilton and George V. Zito, for a Star Position Determination Circuit, filed concurrently with the present application on July 29, 1966.

The system is inherently a two-axis sensing device and thus, by utilizing reference output data from y axis generator 31, the system could be used for two-axis stabilization. Three-axis stabilization would require more than one tracker.

Operation Light impinging on photocathode surface 9 produces an electron stream forming an electron image spatially identical to the photoimage on the outer surface of photocathode 9. In general, owing to pointing error, the photoimage will not fall on the center of photocathode 9 but rather will be displaced some radial distance on the photocathode 9 thereby producing an off axis electron stream. Magnetic deflecting scanning fields are applied to the electron stream by deflection coils 23, 24, 27, and 29 such that the ofl axis electron stream is eventually defiected the right amount to be introduced into aperture 21 producing an output signal across load resistor 47. The output signal developed across load resistor 47 is then applied to a signal processing system 49 which combines it with reference output data from scanning generator 25 such that an error signal is produced having a polarity and magnitude corresponding to the direction and extent the photoimage is displaced from the y axis of the photocathode 9.

Although only one embodiment of the invention has been illustrated and described, various changes in the form and relative arrangements of the parts, which will now appear to those skilled in the art may be made without departing from the scope of the invention. Reference is, therefore, to be had to the appended claims for a definition of the limits of the invention.

What is claimed is:

1. A photomultiplier tube comprising an envelope having a longitudinal axis, a photocathode within said envelope coaxially supported about said axis for emitting an electron stream in response to a photoimage impinging thereon, an electron multiplier mounted within said envelope comprising continuous wall means of secondary electron emissive material defining a path, one end of said wall means defining the entrance end to said path and the other end of said wall means defining the exit end from said path, means for deflecting the electron stream emitted by said photocathode to form at said entrance end an electron image of said photoimage, means for applying a potential difference at said entrance end and said exit end to provide a potential gradient along said electron multiplier without applying potentials in between said entrance end and said exit end thereby minimizing the number of conductors extending through said envelope, and a collector electrode mounted adjacent said exit end for providing an output signal representing the presence of a photoimage on said photocathode.

2. An image dissector photomultiplier tube comprising a photocathode, an electron multiplier having an entrance end and an exit end, means for positioning said entrance end to receive an electron stream emitted from a small segment of said photocathode, scanning means for magnetically deflecting electron stream emitted from other segments of said photocathode toward said entrance end in a progressive manner such that electron streams emitted by said other segments periodically enter said entrance end, said electron multiplier including continuous wall means of secondary electron emissive material between said entrance end and said exit end defining an electron path, and means for applying a potential difference between said entrance end and said exit end to establish a uniform electric field along the central axis of said electron path.

3. A photomultiplier tube comprising an envelope, a photocathode within said envelope for emitting an electron stream in response to a photoimage impinging thereon, a tubular electron multiplier mounted within said envelope having an entrance end and exit end, said entrance end being positioned to receive an electron stream emitted by said photocathode, said tubular electron multiplier having a continuous inner surface of secondary electron emissive material, means for applying a potential between the ends of said inner surface to produce a uniform voltage gradient across said surface thereby providing a uniform electric field along the axis of said tube.

4. A photomultiplier tube as defined by claim 3, wherein said tubular electron multiplier is slightly curved for increasing electron wall collisions.

5. A photomultiplier tube comprising an envelope, a photocathode for emitting an electron stream forming an electron image spatially identical to a photoimage impinging on said photocathode, a tubular electron multiplier, said electron multiplier having a lateral dimension substantially smaller than the lateral dimension of said photocathode, an envelope enclosing said photocathode and said electron multiplier, the portion of said envelope surrounding said electron multiplier having a lateral dimension substantially smaller than the lateral dimension of the portion of said envelope surrounding said photocathode, said tubular electron multiplier having an entrance end and exit end, said entrance end being positioned to receive electrons emitted by said photocathode, means for applying a potential difference between said entrance end and said exit end to produce a uniform electric field along the axis of said tube, a collector electrode mounted adjacent said exit end for providing an output signal representing the presence of a photoimage on said photocathode.

6. A photomultiplier tube comprising a photocathode for emitting an electron stream forming an electron image spatially identical to a photoimage impinging on said photocathode, a tubular electron multiplier having a continuous inner surface of secondary electron emissive material, an envelope enclosing said photocathode and said electron multiplier, the portion of said envelope surrounding said electron multiplier having a lateral dimension substantially smaller than the portion of said envelope surrounding said photocathode thereby minimizing the lateral cross sectional area of the tube, said electron multiplier having an entrance end and an exit end, said entrance end being positioned to receive an electron stream emitted by said photocathode, means for applying a potential difference at said entrance end and said exit end to produce a uniform voltage drop across said secondary emissive surface without applying potentials in between said entrance end and said exit end thereby minimizing the number of conductors extending through said envelope, and a collector electrode mounted adjacent said exit end for providing an output signal representing the presence of a photoimage on said photocathode.

7. A photomultiplier tube comprising an evacuated envelope having a transparent wall, a semitransparent photocathode coated on the inner surface of said transparent wall responsive to a photoimage impinging on its outer surface for emitting an electron stream, a masking electrode supported within said envelope, said masking electrode having a central aperture, an electrostatic lens for focusing an electron stream emitted by said photocathode to form an electron image of said ph-otoimage in the plan of said masking electrode, said central aperture being positioned to receive an electronic stream emitted from a central segment of said photocathode, scanning means for deflecting electron streams emitted from other segments of said photocathode across said central aperture, an electron multiplier supported within said envelope having an input adjacent aperture for receiving electrons passing therethrough, said electron multiplier comprising a tube of insulating material having its inside surface coated with a highly resistant thin film which serves as a secondary electron emitter, means for applying a potential difference between the ends of said film for producing a uniform electric field along the axis of the tube, a collector electrode mounted within said envelope adjacent the output of said electron multiplier responsive to the output of said multiplier for providing an output signal indicating the presence of a photoimage on the outer surface of said photocathode, said scanning means including means for providing reference output data, and conductor means for receiving said output signal and said reference output data for feeding said output signal and said reference output data to a signal processing system for providing an error signal having a polarity and magnitude corresponding to the direction and magnitude the detected photoimage is displaced from the center of a reference axis on said photocathode.

References Cited UNITED STATES PATENTS 2,459,778 1/1949 Larson 178-72 3,243,628 3/1966 Matheson 313-403 2,185,172 1/1940 Bruche et a1 313 X 2,841,729 7/1958 Wiley 313104 3,128,408 4/1964 Goodrich et a1 313-103 3,244,922 4/1966 Wolfgang 313-103 X JOHN W. CALDWELL, Primary Examiner. DAVID G. REDINBAUGH, Examiner.

T. A. GALLAGHER, R. L. RICHARDSON,

Assistant Examiners. 

